Imparting a pearlescent luster, metallic luster and/or multi-color effects approaching iridescent can be achieved using a nacreous or pearlescent pigment which comprises a metal oxide-coated platelet. These pigments were first described in U.S. Pat. Nos. 3,087,828 and 3,087,829, and a description of their properties can be found in the Pigment Handbook, Vol. I, Second Edition, pp. 829-858, John Wiley & Sons, N.Y. 1988.
The oxide coating is in the form of a thin film deposited on the surfaces of the platelet. The oxide in most wide spread use at present is titanium dioxide. The next most prevalent is iron oxide while other usable oxides include tin, chromium and zirconium oxides as well as mixtures or combinations of oxides.
The coating of the metal oxide on the platelet must be smooth and uniform in order to achieve the optimum pearlescent appearance. If an irregular surface is formed, light scattering occurs, and the coated platelet will no longer function as a pearlescent pigment. The metal oxide coating must also adhere strongly to the platelet or else the coating will be separated during processing, resulting in considerable breakage and loss of luster.
During the preparation of these coatings on the platelets, particles which are not attached to the platelet may form. These small particles cause light scattering and impart opacity to the pigment. If too many small particles are present, the pearlescent appearance may be reduced or lost. The addition of these metal oxide coatings to a platelet so that the luster, color and color homogeneity are maintained can be a difficult process, and to date, the only platy substrate which has achieved any significant use in commerce is mica.
A wide variety of other platy materials have been proposed for use as a substrate for forming these pearlescent pigments. These include non-soluble inorganic materials such as glass, enamel, china clay, porcelain, or other silicaceous substances, metal objects and surfaces of organic polymer materials such as polycarbonate. See, e.g., U.S. Pat. Nos. 3,123,485; 3,219,734; 3,616,100; 3,444,987; 4,552,593; and 4,735,869. While glass has been mentioned as a possibility on many occasions, for instance in U.S. Pat. No. 3,331,699, commercial pearlescent products made using glass have been primarily for cosmetic applications in which relatively large glass platelet substrates have been coated.
Aforementioned U.S. Pat. No. 3,331,699 discloses that glass flakes may be coated with a translucent layer of particles of a metal oxide having a high index of refraction, such as titanium dioxide, provided there is first deposited on the glass flakes a nucleating substance which is insoluble in the acidic solution from which the translucent layer of metal oxide is deposited. The glass flakes as disclosed therein are on order of 1.0 to 5.0 microns in thickness, and varying in the size of the major dimension from about 10 microns to about 400 microns, with at least 50 percent below 75 microns and about 85 percent below 150 microns. An example from this patent sets forth the following glass flake size distribution.
SieveSize in MicronsWt. Percent40 to 100mesh149–420microns12.9100 to 200mesh74–149microns32.5200 to 325mesh44–74microns22.0325 to 400mesh37–44microns9.6Through 400meshLess than 37microns23.0
U.S. Pat. No. 5,436,077 teaches a glass flake substrate which has a metal covering layer on which is formed a dense protective covering layer of a metal oxide such as a titanium dioxide. In this patent, the nature of the glass is unimportant as the metallic coating provides the desired appearance and the overcoating of the metal oxide is present to protect the metallic layer from corrosive environments. Examples of a glass flake having an average diameter of 15 microns being plated with silver and then coated with a SiO2 layer are disclosed.
In commonly assigned U.S. Pat. 6,045,914, there is disclosed a method for preparing smooth, uniform coatings of metal oxides on glass flakes which adhere to the glass flakes to yield high quality pearlescent pigments. In accordance with the method disclosed therein, a pearlescent pigment is formed by establishing a hydrous film layer of titanium and/or iron oxides on glass flakes and thereafter calcining the coated flakes provided that the glass flakes employed are C glass flakes and when the hydrous layer is titanium, the procedure is a rutilizing procedure. The glass flakes are disclosed as having have a size and shape mimicking the mica platelets used in the TiO2 and Fe2O3-coated mica pearlescent pigments and thus have an average particle size in the range of about 1 to 250 microns and a thickness of about 0.1-10 microns. More cubic flakes having similar particle sizes and thickness of about 10-100 microns can be utilized, however, the pearlescent effect is significantly diminished due to the low aspect ratio. In all of the examples, however, pigments were made from glass flakes having an average diameter of 100 microns or more. The entire content of U.S. Pat. No. 6,045,914 is herein incorporated by reference.
The manufacture of synthetic platelets such as glass flakes often results in a size distribution of the platelets that can be characterized by Gaussian curves. A particularly useful means of characterizing the size distribution of a mass of synthetic platelets produced and used as substrates for effect pigments is by specifying the platelet size of the lowest 10 vol. %, 50 vol. %, and 90 vol. % of platelets along the Gaussian curve. This classification can be characterized as the D10, D50, and D90 values of the platelet size distribution. Thus, a substrate having a D10 of a certain size means that 10 vol. % of the flake substrate particles has a size up to that value. For example, the present assignee has numerous mica-based effect pigments on the market, in particular used for cosmetics and automotive paint applications. Among these is the LUMINA® mica-based pigment, which has a D10 of 10 microns, a D50 of 22 microns and a D90 of 45 microns. Thus, the size distribution of the LUMINA® mica-based pigment can be described as follows: 10 volume % of the mica platelets have a size of up to and including 10 microns, 50 volume % of the platelets have a size up to and including 22 microns, and 90 volume % of the platelets have a size up to and including 45 microns.
Glass flake-based effect pigments, however, as previously stated, have a substantially larger size. This is reflected in the size distribution. Thus, the present assignee and Nippon Sheet Glass market glass flake-based pigments under the tradenames REFLECKS™ and FIREMIST® based pigments, which have a D10 of 17 microns and D50 of 45 microns for the former and a D10 of 50 microns and D50 of 100 microns for the latter. These pigments are of a particular large size and cannot be effectively used for automotive paints inasmuch as the pigments themselves often protrude from the applied thin paint film adversely affecting the optical properties of the film. Moreover, the large pigments cannot readily pass through the spray apparatus often used to apply the paint. Other glass-based pigments such as a pigment commercialized by Merck under the tradename RONASTAR® are also of a large size, having D10s above 30 and D50s above 65.
In an attempt to manufacture an effect pigment from glass flake that will find acceptance in automotive paints, Nippon Sheet Glass has developed a glass flake substrate having a significantly smaller size distribution than previously formed. This product has a D10 of 8 microns, a D50 of 20 microns and a D90 of 37 microns. Application of TiO2 coatings to produce an effect pigment for automotive paints, however, have not proven successful as the optical properties of the paint films formed from the pigments have lacked luster, depth, and sparkle.